In many new commercial applications for weight reduction, it is required to fill hollow structural members with structural foam as an alternative to metal reinforcement. The use of structural foams enables designers to reduce weight of structural members while maintaining stiffness and structural strength. Methods of reinforcing hollow structural members using two-part, epoxy-resin based systems are known in the art, as illustrated by U.S. Pat. No. 4,995,545, which is incorporated herein by reference in its entirety. This reference discloses a two-part epoxy-based system, where the first part is a mixture of thermosetting resin and expandable microspheres, preferably also containing a filler such as hollow glass microspheres in an amount effective to provide a paste-like consistency. The second part includes a curing agent, which is effective to cross-link and cure the thermosetting resin present in the first part when the two parts are combined. Additionally, the second part contains a filler, such as hollow glass microspheres. Upon mixing, an exothermic reaction takes place, causing the expandable microspheres to increase in size and thereby foaming the composition.
U.S. Pat. No. 4,995,545 suggests that suitable curing agents for the second part of the system are primary polyamines, secondary polyamines, and polyamides (including aliphatic amidoamines). One problem that has arisen with two-part systems described in U.S. Pat. No. 4,995,545 is that, although the second part has good chemical stability at ambient temperatures, the curatives tend to phase separate from the hollow glass microsphere filler material.
In particular, when the material is stored in a 55-gallon drum, the hollow glass microspheres phase separate to form a hard top layer over a bottom liquid layer containing the curatives. Additionally, the curing agent side phase separates when heated and/or when pressure is applied, even when freshly prepared. The liquid curing agents tend to drip, for example, when the curing side agent is heated to about 66° C. (150° F.) and subjected to an application pressure of about 35 kg/cm2 (500 psi). These problems make it quite difficult to dispense or handle the curing side agent by pumping, as would be desirable in an OEM vehicle assembly operation. Therefore, it is desirable to develop a second part that exhibits better storage and processing stability and is pumpable at elevated temperatures and pressures.
Another problem with the prior art two-part systems is the tendency for large voids or holes to develop in the thermosettable composition as the heat generated by the exothermic reaction of the two parts expands the expandable microspheres. The problem is especially pronounced when reactive diluents having relatively low boiling points are present in the first part of the two part system and when a comparatively large mass of the thermosettable composition is being used. The non-uniformity of the resulting foam limits the compression strength and modulus levels which can be attained with such systems. Since these properties are critical when the foam is to be used to reinforce a hollow structural member, it is desirable to have a two part system exhibiting more controlled foaming and a more uniform cell structure.
Obtaining a foamed epoxy resin with an optimum cellular structure is recognized as quite challenging, as there are a number of interrelated parameters which affect the foaming/curing process. For example, the rheology of the epoxy/curative mixture during the rise of the foam is important. As the epoxy resin crosslinks and cures, the mixture becomes more viscous. This is believed to be necessary to retain the cellular structure produced by expansion of the blowing agent. Coalescence and collapse of the foam will occur if the mixture is insufficiently viscous. On the other hand, a mixture which becomes extremely viscous and gels or sets up too quickly may prematurely terminate the foam rise, interfering with the full expansion and density reduction of the foam. Controlling the viscosity of the foam is not straightforward, especially since it will vary with the temperature of the mixture, which often changes significantly during the course of curing/foaming and within the mass of the reacting mixture (the core temperature will often, for example, be much higher than the temperature at the outer edges). Another process parameter related to foam rheology is the epoxy cure rate, which is dependent on the processing temperature as well as the chosen epoxy resin and curing agent. If the epoxy-curative system is fast-reacting with a large exotherm, the cure rate may be too rapid to allow the foam to rise. Further, the excessive heat from a large exotherm can lead to burning or charring of the foam interior. If the epoxy reacts too slowly, the exotherm may not be sufficient to fully activate the blowing agent. Other processing parameters which influence foam quality and cell structure include surface tension and cell nucleation.
A problem with two-component foams of the prior art is that as larger amounts of blowing agent were used to increase foam expansion, the foam structure degraded due to thermal decomposition of the blowing agent and breakdown of the expanded foam cell structure. The inventors have unexpectedly found that a solution to this problem of the prior art is to employ a blowing system comprising at least two blowing agents. One of the blowing agents has a low onset temperature and the other has a high onset temperature. Such a blowing system allows for high levels of expansion while maintaining a good cell structure that provides a lightweight and high strength expanded foam.